Separation processes using molecular sieves



Sept. 20, 1966 A. A. YEO ET AL 3,274,100

SEPARATION PROCESSES USING MOLECULAR SIEVES Filed April 23, 1965 LOW-BO/L/NG FF/ICT/U/V.

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INVENTORS ALAN ARTHUR YEO ROGER TEMPLETON LEWIS MOWLL WW FWM,@MM

ATTORNEYS United States Patent 3,274,100 SEPARATION PROCESSES USING MOLECULAR SIEVES Alan Arthur Yeo and Roger Templeton Lewis Mowll, Sunbury-on-Thames, Middlesex, England, assignors to The British Petroleum Company Limited, London, England, a joint-stock corporation of Great Britain Filed Apr. 23, 1963, Ser. No. 275,128 Claims priority, application Great Britain, May 2, 1962, 16,827/ 62 3 Claims. (Cl. 208-310) This invention relates to separation processes using molecular sieves and particularly to processes for the separation of straight-chain hydrocarbons from petroleum fractions.

It is well known that certain natural and synthetic zeolites have the property of preferentially absorbing certain types of hydrocarbons. These zeolites, known as molecular sieves, have crystalline structures containing a large number of pores of uniform size. In different zeolites, these pores may vary from 4 A. to 15 A. or more in diameter, but in any one zeolite the pores will he of substantially uniform size.

It has previously been proposed to treat petroleum fractions ranging from gasoline to gas-oils and higher with molecular sieves having pore diameters ranging from 4 A. to 15 A. In order to separate straight-chain hydrocarbons from branched-chain and/ or cyclic hydrocarbons a molecular sieve having pore diameters of 5 A. is suitable. Such a process may be used to recover a denormalised fraction, for example gasoline of higher octane number due to the removal of low octane normal paraffins. The absorbed straight-chain material may also be recovered as a product if desired. For example, a process suitable for separating normal parafiins from petroleum fractions boiling within the C -C range is described in our co-pending application No. 3965/61.

According to the present invention a process for separating straight-chain hydrocarbons from petroleum fractions boiling above C and containing molecular sieve deactivating hydrocarbons comprises fractionating the feedstock to obtain a lower-boiling fraction substantially free of deactivating hydrocarbons and a higher-boiling fraction containing substantially all the deactivating hydrocarbons and treating each fraction with a 5 A. molecular sieve in separate systems to extract straight-chain hydrocarbons.

It has been observed that, under equilibrium conditions, the affinity of n-paraflin for molecular sieves increases with increasing molecular weight. Petroleum fractions generally contain small quantities of hydrocarbons which boil outside the nominal boiling range of the fraction, for example up to 0.1% of C hydrocarbons may be present in a nominal C C range gas oil. It is believed that the deactivation rate is at least partially due to the accumulation of high molecular Weight hydro carbons in the sieve pores, these high-boiling hydrocarbons being less readily displaced during the normal desorption stage. In the present application, molecular sieve deactivating hydrocarbons means these high-boiling straight-chain hydrocarbons which may, for example, be C hydrocarbons or higher.

It is desirable to ensure that substantially all the highboiling deactivating hydrocarbons are obtained in the higher-boiling fraction but at the same time to ensure that as much as possible of the lower-boiling, nondeactivating material is obtained in the lower-boiling fraction. Preferably, therefore, the feedstock is fractionated at a point within the range 220 to 300 C., and particularly just below the initial boiling point of n-C hydrocarbons, i.e. in the region of 300 C.

, quantity of sieve involved is much less.

Patented Sept. 20, 1966 By fractionating the feedstock and treating each of the fractions in separate molecular sieve systems according to the present invention, deactivation is substantially limited to the sieve used to treat the higher-boiling fraction. The quantity of sieve required to treat the higher boiling fraction is much less than that which would be required to treat the unfractionated feedstock; also it has been observed that the deactivation rate is substantially independent of the amount of lower-boiling hydrocarbons present in the feed to the sieve, so that the deactivation rate is the same when treating the higher-boiling fraction as when treating the unfractionated distillate. There is, therefore, a saving during regeneration since, although the sieve used for treating the higher-boiling feedstock has to be regenerated after a similar on-stream period as a sieve used for treating unfractionated distillate, the Regeneration may be carried out in the conventional manner by contacting the deactivated sieve with an oxygen-containing gas under controlled conditions to burn off the deactivating material. A further advantage of operating according to the present invention is the increase in yield obtained. This is due partly to the fact that the sieve used for treating the lower-boiling fraction is not deactivated to any great extent and, in addition, to the fact that the sieve has a higher initial capacity when treating the lower-boiling fraction than when treating either the heavier fraction or the unfractionated distillate. Yet another advantage is that optimum conditions may be chosen in each molecular sieve system to suit the boiling range of each fraction whereas in a process for treating a wide boiling range feedstock in a single molecular sieve system, it is diificult to choose conditions which are suitable both for treating the higher boiling materials and the lower boiling materials present in the feedstock.

The method of the present invention may be employed to treat any hydrocarbon feedstocks which contain deactivating hydrocarbons and any appropriate molecular sieve treatments may be used to treat the fractions separated. A particularly suitable molecular sieve treatment for hydrocarbon feedstocks boiling in C C range is described in our co-pending U.K. application No. 3,965 61. This process is isothermal and comprises contacting the fraction with a 5 A. molecular sieve in a first stage to absorb straight-chain hydrocarbons, contacting the sieve with a purging medium in a second stage to remove material absorbed on the surface of the sieve or held interstitially between sieve particles, desorbing the absorbed hydrocarbons in a third stage using either n-pentane or n-butane as deso-rbing medium, all three stages being in the vapour phase, the pressure in the desorption stage being equal to, or greater than, the pressure in the absorption stage, and recovering the desorbed straight-chain hydrocarbons in a purity of at least weight. Preferred process conditions include a temperature within the range 300-450 C., preferably 350-390 C., and a pressure within the range 0-300 p.s.i.g. preferably -150 p.s.i.g. In the present invention, preferred conditions for treating the lower boiling fraction are 380 C. and p.s.i.g. and for treating the higher boiling fraction 380 C. and 125 p.s.i.g.

The invention is illustrated by means of the following comparative example:

Example A 180340 C. petroleum distillate was fractionated to give a fraction boiling below a nominal temperature of 243 C., this fraction containing 0.01% weight of sulphur, and a fraction boiling above a nominal 243 C., this fraction containing 0.025% weight of sulphur. Each fraction was treated for the recovery of normal parafiins with 1300 g. of 5 A. molecular sieve in separate threestage cyclic systems under the conditions set out in the following table:

4, Thus an increase in n-paraffin yield of 6630 grams was obtained by operating according to the present invention.

Lower boiling fraction Higher boiling fraction Stage Agent Tempera- Pressure, Space Period, Tcmpera- Pressure, Space Period, ture, C. p.s.i.g. Velocity mms. ture, C. p.s.i.g. Velocity mins.

. Feed 0.6 LHSV 0.6 LHSV.-. Absorption... {Nitrogenfln} 350 150 120 Gnsvm 6 380 125 120 a Purge Nitrogcnufl 350 150 120 GHSV 6 380 125 120 GHSV 6 Desorption..- n-Pentaue- 350 150 1.0 LHSV 12 380 125 1 2 LHSV..- 12

For purposes of comparison a sample of the original We claim:

unfractionated distillate was treated with 2600 grams of 5 A. molecular sieve under the conditions indicated above 15 for the higher boiling fraction.

The variation of sieve capacity in each of the three systems with time is indicated in the accompanying graph. It is seen from the graph that substantially no sieve deactivation took place during the treatment of the lower- 20 boiling fraction over a period of 300 hours. During the treatment of the higher-boiling fraction the sieve was deactivated progressively and, moreover, the initial capacity of the sieve Was lower than when treating the lowerboiling fraction. tionated distillate, the initial capacity and the decay rate of the sieve were observed to be the same as during the treatment of the higher-boiling fraction.

The yields of n-paraffins obtained from the three systems were as follows:

Original distillate 26,520

During the treatment of the unfrac- 25 1. A process for separating straight-chain hydrocarbons from petroleum fractions boiling above C and containing molecular sieve deactivating hydrocarbons comprising fractionating the feedstock at a temperature within the range of 220-300 C. to obtain a lower-boiling fraction substantially free of deactivating hydrocarbons and a higher-boiling fraction containing substantially all the deactivating hydrocarbons and treating each fraction with a 5 A. molecular sieve in separate systems to extract straight-chain hydrocarbons.

2. A process as claimed in claim 1 for the treatment of teedstocks comprising petroleum fractions boiling within the C -C range.

3. A process as claimed in claim 1 wherein the feedstock is fractionated at a temperature of the order of 300 C.

References Cited by the Examiner UNITED STATES PATENTS 2,935,467 5/1960 Fleck et al 208-310 ALPHONSO D. SULLIVAN, Primary Examiner. 

1. A PROCESS FOR SEPARATING STRAIGHT-CHAIN HYDROCARBONS FROM PETROLEUM FRACTIONS BOILING ABOVE C10 AND CONTAINING MOLECULAR SIEVE DEACTIVATING HYDROCARBONS COMPRISING FRACTIONATING THE FEEDSTOCK AT A TEMPERATURE WITHIN THE RANGE OF 220-300*C. TO OBTAIN A LOWER-BOILING FRACTION SUBSTANTIALLY FREE OF DEACTIVATING HYDROCARBONS AND A HIGHER-BOILING FRACTION CONTAINING SUBSTANTIALLY ALL THE DEACTIVATING HYDROCARBONS AND TREATING EACH FRACTION WITH A 5 A. MOLECULAR SIEVE IN SEPARATE SYSTEMS TO EXTRACT STRAIGHT-CHAIN HYDROCARBNS. 